Dietary Restriction of Sodium as a Means of Reducing Urinary Cystine
Cystinuria is an autosomal recessive disorder manifested by impaired renal tubular reabsorption of cystine and frequent occurrence of urinary calculi. Medical therapy is based upon urinary dilution and reduction in cystine excretion. Increasing fluid intake and decreasing animal protein (methionine) intake are often recommended dietary treatments. Sodium bicarbonate, the most frequently used urinary alkalinizer is another suggested treatment as it can improve cystine solubility. Unfortunately, this latter treatment may be inappropriate as there is evidence to suggest that cystine excretion may be sodium dependent (NEJM 315:1120 (1986)). To further investigate the relationship between dietary sodium and urinary cystine, five patients with cystinuria and a mean 17-year history of recurrent cystine stones were studied. Initially, all patients were on unrestricted diets (high sodium diet) when they completed the first set of 24-h urine collections and corresponding diet records. Two patients were on sodium bicarbonate. Dietary therapy was then introduced and patients were advised to follow a low sodium diet (87 mmol/day or 2 g Na/day). Protein (methionine) intake was not altered and alkalinizing agents were discontinued. After following the low sodium diet for six weeks, the 24-h urine collections and diet records were repeated. Table 1 summarizes the mean results.